A “real world” power supply consisting of a 2S Thunder Power ProLite Li-Po regulated by a Tech Aero Flex Reg PLR5-E was used along with a Hangar 9 voltmeter with selectable load to simulate and evaluate voltage losses in commonly used servo extensions and connectors (at left). Custom servo wire extension uses magnet wire and common positive and negative leads to save weight can be employed on performance models (above).
of a quick bit of math to break out the resis- tance of wiring and the connectors, and “nor- malize” wiring resistance for 10-inch lengths. To my eye, there are several standout ob- servations/conclusions from this data: 1. The data is generally linear, and can be extrapolated for higher amp loads and AWG sizes not specifically tested. This behavior is generally consistent with reference values, given an allowance for the accuracy of equip- ment used. 2. Heavy duty wiring (22 AWG) and ap- purtenant connectors have roughly half the voltage drop when compared to standard wiring (26 AWG). Put another way, a 22 AWG extension carrying a 2-amp load expe- riences the same voltage drop as a 26 AWG extension carrying a 1-amp load (but the 22 AWG is only 1.5 times the weight). 3. At low current draws (<0.5 amps), the
voltage drop attributable to even standard wiring (26 AWG) and connectors is relative- ly low, and even when summed, is unlikely to exceed 0.10 volts. In smaller planes where individual servos rarely demand more than 0.50 amps and long wiring runs and multi- ple extensions are rarely needed, the voltage at the servo will be minimally increased (~0.05 volts, or 1% of operating voltage) us- ing heavier 22 AWG wiring. 4. At higher current draws (2.0 amps and greater), the voltage drop on standard con- nectors and 26 AWG wiring can start to add up quite quickly, dropping the output volt- age from the Rx by approximately 0.60 volts
at a 2-amp load with a single 36-inch exten- sion. 0.60 volts is typically 10% of the oper- ating voltage, resulting in approximately 10% less servo torque and speed. Using a 22 AWG heavy duty 36-inch extension will cut the voltage drop approximately in half, pre- serving 0.30 volts at the servo. The greatest opportunity to preserve volt-
age is between the power source (BEC or air- borne battery) and the receiver as it is at this point that the greatest amp loads are experienced (the sum of all servos plus the Rx itself). Whenever possible, connections and long wiring runs between the power source and Rx should be avoided. In instal- lations where a voltage regulator (Vreg) is used, the Vreg should be placed close to the Rx to ensure the desired output voltage from the Vreg is realized at the Rx. The only ex- ception to this being if there is concern for interference from the Vreg to Rx, which should not be an issue with modern 2.4 GHz radio systems and a quality Vreg. Addition- ally, in the instance when parallel (load sharing) power sources are used, the num- ber of connections and wiring lengths of each source should be equal to eliminate a greater voltage drop in one source which could potentially place a greater load on one of the sources. With larger planes, multiple servos are of-
ten located in the tail and use separate channels for ease of radio setup and pro- gramming options. Running multiple 36- inch, 48-inch, or longer extensions to each
Table 2 — Voltage Losses LOAD (in Amps)
Extension Type
Per 10″ length of heavy duty wire, 22 AWG Per 10″ length of standard wire, 26 AWG Per heavy duty servo connector Per standard servo connector
FLYING MODELS
0.00 0.00 0.00 0.00 0.00
0.50 0.02 0.03 0.01 0.03
1.00 2.00 0.03 0.07 0.06 0.13 0.03 0.05 0.04 0.12
servo can very rapidly add weight. Referring back to observation/conclusion number 2, there is an opportunity to save weight using a custom made extension with common pos- itive and negative leads. For example, an 18-inch JR brand heavy duty 22 AWG ex- tension lead weighs 7.5 grams while an 18- inch standard 26 AWG lead weighs 5.2 grams. The 22 AWG extension has twice the current capacity (for an equal drop in volt- age) but is not twice the weight. I recently finished installing a pair of ser-
vos in the stabilizers of an electric pattern plane and needed to connect the servos to the Rx located approximately 40 inches away. Finding the standard 36-inch length extension a bit short, and the next size up 48-inch extension a bit long, I set about building an extension(s) at the exact length I needed for a tidier installation. Of course I took the opportunity to reduce some weight using common positive and negative leads for the two elevator servos. I started with a pair of 6-inch 22 AWG ex- tensions and “borrowing” from competition foamies, I used 22 AWG magnet wire for the positive and negative leads, and 30 AWG magnet wire for the signal leads. In essence, I cut the 6-inch leads in half so I could insert (solder) approximately 35 inches of magnet wire extensions. At each end, the two posi- tive leads were joined to the common positive extension, and the two negative leads were joined to the common negative extension. Each signal lead was joined to unique exten- sions. The end result is that the completed extension is a bundle of six wires at the Rx and servos, but only four wires in the middle ~85% of the extension. The finished exten- sion weighed only 16 grams, approximately 25% lighter than a pair of standard 26 AWG extensions at 22 grams (for 40-inch length). The extension was installed securely at
each end and supported in multiple places in the middle. All “flexing” of the extension occurs within the silicone insulated braided wiring which is far more durable. I would not use the solid strand wiring in a glow/gas powered plane.
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